The present invention relates to the field of electronic circuits, and, more particularly, to low drop-out (LDO) type linear voltage regulators, namely low serial voltage drop-out regulators.
Low drop-out (LDO) linear voltage regulators, such as low serial voltage drop-out regulators, may be used in several applications. In particular, such regulators may be used in mobile telephones to deliver a regulated voltage to radio transmission/reception circuits from a battery.
By way of example, a standard linear regulator 10 according to the prior art whose output delivers a regulated voltage Vout to a load Z is shown in FIG. 1. The load Z represents, for example, several radio circuits in a mobile telephone. The regulator 10 is electrically powered by a voltage Vbat delivered by a battery 1 and includes a differential amplifier 2 whose output drives a gate G of a P-channel metal oxide semiconductor (PMOS) regulation transistor 3. The transistor 3 is generally a transistor with low serial resistance in the conductive or on state (drain-source resistance RdsON), and it receives the voltage Vbat at its source S. A drain D of the transistor 3 is connected to the output of the regulator 10 and to the anode of a capacitor Cst for filtering and stabilizing the voltage Vout. This capacitor Cst is parallel-connected with the load Z.
The amplifier 2 receives a reference voltage Vref at its negative input and a feedback voltage Vfb at its positive output. The voltage Vfb is a fraction of the voltage Vout provided to the input of the amplifier 2 by a divider bridge including two resistors R1, R2.
Operation of a regulator of this kind, which is well known to those skilled in the art, includes modulation of the gate voltage Vg of the transistor 3 using the amplifier 2. This is done based upon a difference between the voltage Vfb and the reference voltage Vref, which the amplifier maintains at about 0 V. When the voltage Vg is smaller than the value Vbatxe2x88x92Vtp, the transistor 3 is on because its gate-source voltage Vgs is higher than the threshold voltage Vtp. When the voltage Vg is higher than Vbatxe2x88x92Vtp, the transistor is off. In a stabilized mode, the voltage Vout is regulated in the neighborhood of its nominal value Voutnom, which is equal to (R1+R2) Vref/R2.
A typical embodiment of the amplifier 2 according to the prior art is shown in FIG. 2. The amplifier 2 includes a differential stage 5 which receives the voltages Vref and Vfb as inputs and is biased by a current generator 6. The output of the differential stage 5 drives the gate of an N-channel MOS (NMOS) transistor 7 connected between the output node of the amplifier 2 and ground. The transistor 7 is biased at its drain D by a current generator 8. The output node of the amplifier is connected to the power supply voltage Vbat by a gate resistor Rg, which determines the gain of the amplifier and the maximum current that can be delivered at the output. Thus, according to the value of the signal delivered by the differential stage 5, the transistor 7 draws the output of the amplifier 2 to ground or the resistor Rg draws the output upwards, namely toward the voltage Vbat.
In an application such as supplying power in a mobile telephone, it is important that the regulation amplifier consume as little electricity as possible to maintain the charge of the battery. To this end, the gate resistance Rg is chosen such that it has a high value (e.g., 100 Kxcexa9) to limit the current flowing in the output stage. At the same time, the currents delivered by the generator 6, 8 are calibrated appropriately. Generally, the choice of the resistance Rg and of the bias currents is the result of a compromise between the need to efficiently drive the transistor 3, which generally has a high parasitic gate capacitance, and the need for low consumption.
Such consumption is typically in the range of 50 to 200 microamperes, i.e., it is acceptable per se when the battery is properly charged, and allows the regulator to work in a stabilized mode. Yet, the present invention is based on the assumption that this consumption is too high when the battery voltages Vbat become low and are below the nominal value Voutnom of the output voltage. Such a drop in the voltage Vbat below the nominal voltage Voutnom may be temporary and due to high current consumption, or it may be due to the fact that the battery has become discharged.
Turning now to FIGS. 3A, 3B, and 3C, according to observations and conclusions that form an integral part of the present invention, the passage of the voltage Vbat below the value Voutnom at an instant tA (FIG. 3A) results in the feedback voltage Vfb being lower than Vref at the input of the amplifier 2. This voltage is unbalanced and makes the gate voltage Vg drop to ground to compensate for the imbalance (FIG. 3B). The regulation transistor 3 is continually on, the voltage Vout becomes substantially equal to the voltage Vbat (FIG. 3), and the regulator 10 works in the follower mode. Since the output node of the amplifier 2 is grounded, it can be seen in FIG. 2 that the consumption in the gate resistor Rg is at the maximum.
Thus, the amplifier consumes current unnecessarily when the regulator works in the follower mode. This is because the regulation transistor is continually on and the output voltage Vout can no longer be regulated.
An object of the present invention is to provide a voltage regulator which overcomes the above drawback, for example, by switching the regulation amplifier into a low consumption standby mode while keeping the regulation transistor in the on state.
This and other objects, features, and advantages according to the present invention are provided by a, voltage regulator including a regulation MOS transistor and an amplifier whose output drives a gate of the regulation MOS transistor based upon a difference between a reference voltage and a feedback voltage. The voltage regulator may also include a circuit or means to make the amplifier change over into a standby mode with low current consumption when the difference between the supply voltage and the output voltage of the regulator is below a first threshold. This is done while keeping an electrical potential at the gate of the regulation MOS transistor at a value that keeps the regulation MOS transistor in the on state.
More specifically, the voltage regulator may include a comparator for comparing the supply voltage and the output voltage of the regulator and delivering a standby signal to the amplifier when the difference between the supply voltage and the output voltage of the regulator is below the first threshold. Also, the comparator provides a switch-over hysteresis and cancels the signal for putting the amplifier on standby when the difference between the supply voltage and the output voltage of the voltage regulator is higher than a second threshold, where the second threshold is higher than the first threshold.
Additionally, the amplifier may include a resistor connecting the output of the amplifier to the supply voltage. Further, a switch may be series-connected with the resistor and may be open when the amplifier is put on standby. Otherwise, this switch is closed. The amplifier may also include current sources that switch to low current mode when the amplifier is put on standby.
In addition, the amplifier may include a switch driven by a standby-setting signal to connect the gate of the regulation MOS transistor to an electrical potential making the regulation MOS transistor conductive when the amplifier is put on standby. The amplifier may also include a stage for biasing the gate of the regulation MOS transistor when the amplifier is on standby. The stage biases the gate with a voltage that is set so that the gate-source voltage of the regulation MOS transistor is close to the threshold voltage of the regulation MOS transistor. The electrical supply of the amplifier may be eliminated in the standby mode by a switch.
A mobile telephone according to the invention includes at least one radio circuit, a battery, and a voltage regulator as described above for powering the at least one radio circuit from the battery.
A method aspect of the invention is for managing the power available in a battery powering a load using a voltage regulator. The voltage regulator includes a regulation MOS transistor and an amplifier whose output drives a gate of the regulation transistor based upon a difference between the reference voltage and a feedback voltage. The method includes monitoring the difference between the supply voltage and the output voltage of the regulator and switching the amplifier to a standby mode providing low current consumption when the difference between the supply voltage and the output voltage of the regulator is below a first threshold. This is done while keeping the gate of the regulation MOS transistor at a potential that keeps the regulation MOS transistor in the on state.
More specifically, the amplifier may be reactivated when the difference between the supply voltage and the output voltage of the regulator is higher than a second threshold, where the second threshold is higher than the first threshold. The consumption of the amplifier may be reduced in standby mode by disconnecting the output node of the amplifier from the supply voltage, diminishing the current delivered by current sources internal to the amplifier, or disconnecting the supply voltage. Further, when the amplifier is put on standby, it is advantageous to apply a gate voltage to the gate of the regulation MOS transistor where the gate voltage is set so that the gate-source voltage of the regulation transistor is close to its threshold voltage.